i read on someone's blog that she passed through a sophisticated camera/counting device used by UFPJ. Any word on that? ON the UFPJ site, it says that the NYPD command center confirmed the .5 million figure.
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I don't know any specifics about UFPJ's (or the NYPD's) methodology and the precise tech they employed but here are some examples of the sort of systems currently available - some version of which was undoubtedly used:
good pop science article
<http://www.wired.com/wired/archive/11.06/crowd_spc.html
>
comparative study of people counting systems (everything you never wanted to know, complete with the mathematics to prove it)
<http://www.prip.tuwien.ac.at/~krw/papers/2001/OeAGM/Schloegel.pdf
>
overview of thermal imagers
product example
<http://www.infodev.ca/AN/vehicle/VideoAN.shtml >
....
Imaging-based counting systems are reportedly very accurate (properly used) so any estimates based upon results from this tech are probably dead-on within a small margin of error.
Good god, I'm such a technosphere-slut.
.d.